Input protection circuitry is an essential part of modern integrated circuits, particularly in the area of field effect transistor based devices. Extremely small delicate device structures are very sensitive to the high voltages which may be generated by the electrostatic charge developed in the human body. During installation of integrated circuits into products, these electrostatic discharges may destroy integrated circuits and thus require expensive and tedious repairs on fully manufactured devices which could have been avoided by providing a mechanism for dissipation of the electrostatic discharge on the integrated circuit. This problem is particularly acute in complimentary metal oxide semiconductor field effect transistor type integrated circuits. In high density CMOS devices, an extremely high voltage may be developed by electrostatic discharge which easily destroys the very thin gate oxides and very short channel devices of the integrated circuit.
To protect against these over-voltage conditions it has been proposed that a silicon controlled rectifier (SCR) structure compatible with complementary metal oxide semiconductor (CMOS) processing be utilized. This structure is shown in copending application Ser. No. 213,499, filed June 27, 1988 and assigned to the assignee of this application. This structure provides excellent protection but the SCR protection device has a very high threshold voltage. This threshold voltage is commonly on the order of 70-80 volts. It has been found in many present day circuits that a 70-80 volt threshold is inadequate to protect some devices on the integrated circuit. Therefore, it is highly desirable to provide an input protection structure which greatly reduces the threshold voltage of the input protection device, is simple and is compatible with CMOS processing.